1. Field of the Invention
The present invention relates to a carbon monoxide concentration reduction apparatus, a method of reducing the concentration of carbon monoxide, and a carbon monoxide selective oxidation catalyst. More specifically the present invention pertains to a carbon monoxide concentration reduction apparatus that reduces the concentration of carbon monoxide included in a hydrogen rich gas, a corresponding method of reducing the concentration of carbon monoxide, and a carbon monoxide selective oxidation catalyst used therefor.
2. Description of Related Art
Proposed carbon monoxide concentration reduction apparatuses for reducing the concentration of carbon monoxide included in a hydrogen rich gas utilize a ruthenium catalyst carried on a carrier, such as alumina (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 8-133701, No. 8-133702, and No. 8-217406). When the hydrogen rich gas and a predetermined quantity of oxygen are fed into any of these apparatuses, the ruthenium catalyst accelerates a carbon monoxide selective oxidation reaction, which oxidizes carbon monoxide, in preference to the oxidation of hydrogen and thereby reduces the concentration of carbon monoxide included in the hydrogen rich gas.
Such a carbon monoxide concentration reduction apparatus is used in a fuel cells system including, for example, polymer electrolyte fuel cells or phosphate fuel cells. The following electrochemical reactions proceed in these fuel cells:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
2H++2exe2x88x92+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
Equation (1) shows the reaction proceeding on the anodes of the fuel cells, Equation (2) shows the reaction proceeding on the cathodes of the fuel cells, and Equation (3) shows the reaction proceeding in the whole fuel cells. As clearly understood from these equations, for the progress of the reaction in the fuel cells, it is required to feed a supply of a hydrogen-containing gaseous fuel to the anodes and a supply of an oxygen-containing oxidant gas to the cathodes. Carbon monoxide that is present in these supplies of the gases is adsorbed on a platinum catalyst included in the fuel cells and deteriorates its catalytic ability. The air, which is generally used as the oxidant gas, does not contain carbon monoxide to the level that lowers the catalytic ability. The gaseous fuel, on the other hand, generally contains a little quantity of carbon monoxide, which may interfere with the dissociation of hydrogen proceeding on the anodes and lower the performance of the fuel cells.
The presence of carbon monoxide in the gaseous fuel is ascribed to the production of the gaseous fuel by reforming a hydrocarbon. The problem of carbon monoxide described above accordingly arises when the gaseous fuel used is not the gaseous hydrogen of a high purity but the hydrogen rich gas produced by reforming a hydrocarbon. The fuel cells system, which utilizes the reformed gas as the gaseous fuel supplied to the fuel cells, generally has a fuel reformer unit that reforms a hydrocarbon to produce a hydrogen rich gaseous fuel, which is fed to the anodes of the fuel cells. The following shows an example of the reforming reactions to produce the hydrogen rich gas, in which methanol is steam reformed:
CH3OHxe2x86x92CO+2H2xe2x80x83xe2x80x83(4)
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(5)
CH3OH+H2Oxe2x86x92CO2+3H2xe2x80x83xe2x80x83(6)
In the steam reforming reaction of methanol, the dissociation of methanol shown by Equation (4) and the reforming of carbon monoxide shown by Equation (5) simultaneously proceed. As a whole, the reaction shown by Equation (6) occurs to produce a hydrogen rich gas containing carbon dioxide. No carbon monoxide is present in the final stage if these reactions completely proceed. In the actual fuel reformer unit, however, it is practically impossible to shift the reaction of Equation (5) completely to the right. The gaseous fuel reformed by the fuel reformer unit accordingly contains a trace amount of carbon monoxide as a side product.
The steam reforming reaction generally proceeds in the presence of a known reforming catalyst like a Cu-Zn catalyst. In the presence of the reforming catalyst, however, a reverse shift reaction shown by Equation (7) given below proceeds with the steam reforming reaction discussed above, so as to generate a trace amount of carbon monoxide in the reformed gas:
CO2+H2xe2x86x92CO+H2Oxe2x80x83xe2x80x83(7)
The reverse shift reaction shown by Equation (7) produces carbon monoxide from hydrogen and carbon dioxide, which are obtained in the process of the steam reforming reaction. The reverse shift reaction proceeds only slightly, compared with the steam reforming reaction. In the case where an extremely low concentration of carbon monoxide is required, for example, when the reformed gas is used as a supply of gaseous fuel fed to the fuel cells, however, carbon monoxide produced by the reverse shift reaction is not negligible but may have a significant influence.
The carbon monoxide concentration reduction apparatus is accordingly used to reduce the concentration of carbon monoxide included in the gaseous fuel, prior to the supply of the gaseous fuel to the fuel cells. In the carbon monoxide concentration reduction apparatus, the selective oxidation of carbon monoxide proceeds in preference to the oxidation of hydrogen, which is affluently present in the reformed gas as mentioned above. The oxidation reaction of carbon monoxide is shown by Equation (8) given below. The allowable concentration of carbon monoxide included in the gaseous fuel fed to the fuel cells is not greater than several percents in the case of phosphate fuel cells and not greater than several ppm in the case of polymer electrolyte fuel cells. The reformed gas is introduced into the carbon monoxide concentration reduction apparatus including the ruthenium catalyst, and the selective oxidation of carbon monoxide shown by Equation (8) proceeds in the apparatus. This lowers the concentration of carbon monoxide included in the reformed gas and ensures the supply of the gaseous fuel having a sufficiently low concentration of carbon monoxide to the fuel cells.
CO+(xc2xd)O2xe2x86x92CO2xe2x80x83xe2x80x83(8)
The effective temperature range of the ruthenium catalyst, in which the carbon monoxide selective oxidation reaction is sufficiently accelerated, is about 140 to 200xc2x0 C. The carbon monoxide concentration reduction apparatus with the ruthenium catalyst incorporated in the fuel cells system may not sufficiently lower the concentration of carbon monoxide included in the gaseous fuel fed to the fuel cells. When the temperature in the carbon monoxide concentration reduction apparatus becomes lower than the effective temperature range, the catalytic activity lowers and does not sufficiently accelerate the oxidation of carbon monoxide. This results in the insufficient reduction of the concentration of carbon monoxide. When the temperature in the carbon monoxide concentration reduction apparatus becomes higher than the effective temperature range, on the other hand, hydrogen that is affluently present in the gaseous fuel is oxidized. This interferes with the selective oxidation of the trace amount of carbon monoxide co-existing in the gaseous fuel. In order to lower the concentration of carbon monoxide sufficiently, it is required to regulate the internal temperature of the carbon monoxide concentration reduction apparatus according to the amount of the reformed gas, which is subjected to the selective oxidation of carbon monoxide and to make the selective oxidation of carbon monoxide proceed in the effective temperature range mentioned above.
Especially in the event that the load to be processed in the carbon monoxide concentration reduction apparatus (that is, the quantity of the reformed gas fed to the carbon monoxide concentration reduction apparatus) remarkably varies, it is difficult to keep the internal temperature of the carbon monoxide concentration reduction apparatus in the effective temperature range. For example, when the fuel cells that receive the supply of gaseous fuel having the reduced concentration of carbon monoxide are used as a power source for driving a vehicle, the loading drastically varies. The variation in loading significantly varies the quantity of the reformed gas to be processed in the carbon monoxide concentration reduction apparatus. This makes it difficult to regulate the internal temperature of the carbon monoxide concentration reduction apparatus. An abrupt increase in loading leads to a remarkable increase in load to be processed in the carbon monoxide concentration reduction apparatus and may thus abruptly raise the internal temperature. Similarly an abrupt decrease in loading leads to a remarkable decrease in load to be processed in the carbon monoxide concentration reduction apparatus and may thus abruptly lower the internal temperature. In the event that the internal temperature of the carbon monoxide concentration reduction apparatus is deviated from the desirable temperature range due to the variation in loading, the above problems arise to interfere with the effective reduction of the concentration of carbon monoxide included in the reformed gas. It is accordingly desirable that the carbon monoxide selective oxidation catalyst has a wider effective temperature range, in order to keep the activity of the selective oxidation of carbon monoxide in a sufficient level under the condition of a relatively large variation in loading of the fuel cells.
The driving temperature of the fuel cells, which receive the supply of gaseous fuel fed from the carbon monoxide concentration reduction apparatus is about 80 to 100xc2x0 C. in the case of polymer electrolyte fuel cells. When the temperature of the gaseous fuel fed from the carbon monoxide concentration reduction apparatus to the fuel cells is higher than the driving temperature of the fuel cells, the direct supply of the gaseous fuel from the carbon monoxide concentration reduction apparatus to the fuel cells disadvantageously increases the internal temperature of the fuel cells to an undesirable level. In the case where the temperature of the oxidation reaction proceeding in the carbon monoxide concentration reduction apparatus (that is, the regulated temperature to enable the carbon monoxide selective oxidation catalyst to sufficiently accelerate the reaction) exceeds the driving temperature range of the fuel cells, a heat exchange unit should be disposed in the flow path of the gaseous fuel that connects the carbon monoxide concentration reduction apparatus to the fuel cells. The heat exchange unit sufficiently lowers the temperature of the gaseous fuel, prior to the supply of the gaseous fuel to the fuel cells. The heat exchange unit, however, makes the piping layout rather complicated and undesirably increases the size of the whole system.
As described above, it is desirable that the catalyst used for accelerating the selective oxidation reaction of carbon monoxide has a wider effective temperature range, in which the selective oxidation of carbon monoxide is sufficiently accelerated, in case of a possible variation of the loading. It is especially preferable that the lower limit of the effective temperature range is closer to the driving temperature of the fuel cells. In the case of the ruthenium catalyst conventionally used, however, the lower limit of the effective temperature range, in which the concentration of carbon monoxide is sufficiently reduced, is about 140xc2x0 C. as described previously. The ruthenium catalyst can not sufficiently accelerate the selective oxidation reaction of carbon monoxide in the driving temperature of the fuel cells, which is about 100xc2x0 C.
The object of the present invention, which is actualized in the form of a carbon monoxide concentration reduction apparatus, a method of reducing concentration of carbon monoxide, and a carbon monoxide selective oxidation catalyst, is thus to extend an effective temperature range, in which the activity of a carbon monoxide selective oxidation reaction is kept sufficiently high, and especially to make the lower limit of the effective temperature range closer to the driving temperature of fuel cells.